1. Technical Field of the Invention
The present invention relates to modulation systems, and more particularly, to a modulation system providing increased data rates that is backward compatible with existing FSK modulation systems.
2. Description of Related Art
The two level NRZ (Non-Return to Zero) frequency shift key (FSK) modulation technique used by some control channels within mobile radio communication systems is well defined, and many existing mobile radio products have been manufactured and sold that recognize this modulation and messaging protocol. However, as the demand for mobile radio communication services increase, and the variety of available functionalities associated with these services increase, existing systems are experiencing a problem with the group and/or logical IDs that are possible within the bit structure of existing systems. The capabilities and range of existing systems have been temporarily extended by utilizing bit stealing techniques within the broadcast channel (BCH) codes or other similar techniques. However, a more permanent solution requires an increase in the number of bits provided by the modulation scheme.
In order to preserve existing hardware infrastructure, it is highly desirable that any new system providing increased bits (data rates) should not affect the existing product base of mobile radio communications gear. Changes may occur within system level units, such as, the base station infrastructure. However, the mobile radio terminals must preferably remain unchanged.
One modulation technique known as .PI./2-DBPSK (differential binary phase shift key) has been proposed for use within a narrowband (12.5 kHz) system that provides an improved spectrum over a reduced deviation FSK technique. The .PI./2-DBPSK technique involves sending phase changes much like a .PI./4-DQPSK (differential quadrature phase shift key) except that only one bit is encoded at a time. For instance, a 0 represents a .PI./2 change in a positive direction and a 1 would represent a negative .PI./2 change in the negative direction. This generates a scatter diagram as shown in FIG. 1.
A two level FSK modulation technique performs basically the same function except that the phase transition is along a constant magnitude circle that the constellation represents. The .PI./2 modulation technique takes a more direct path which indicates that it has amplitude variation. The net difference on the receiving radio is that they possess slightly different eye patterns. The assumption is that if FSK and .PI./2-DBPSK both deviate by the same general amount, the frequency shift key receiver will not know the difference between the two, especially if the incoming signal is squared up relative to the zero crossing. The .PI./2-DBPSK system provides a spectrum and bit error rate improvement but does not increase the data rate of the channel. A system providing these improvements would be greatly desired.